Mother to child transmission of HIV-1 occurs in 15-40% of children born to HIV-1 seropositive mothers. It remains unclear why some others transmit HIV-1 to their infants while the majority do not. Studies to date indicate multiple factors associated with maternal-fetal transmission including plasma viremia, maternal immune response, maternal viral phenotypes, use of anti-retroviral agents during pregnancy, and obstetrical. Little is known of the mode of transmission or risk factors associated with transmission in utero. Identification of the role played by the placenta in maternal-fetal transmission of HIV-1 may lead to the development of strategies to prevent transplacental infection of the fetus. It is our hypothesis that: (i) the HIV-1 viral load is higher in the placentas of mothers with high plasma viremia during pregnancy resulting in higher rates of transmission; and (ii) early elimination of HIV-1 infected cells in the PBMCs of newborns results in long-term asymptomatic survivors whereas failure to do so results in the early manifestation of AIDS. In this application, we propose to (i) examine the correlation between PBMC/plasma viral burden during pregnancy and the viral burden of the placenta at the chorioamniotic layer at delivery; and (ii) examine the viral burden of the newborn at the time of delivery and throughout the first six months of life in correlation with the subsequent clinical development of HIV-1 related disease. 1. Determination of viral burden in maternal PBMCs and placental tissue. We will analyze the viral burden in: (i) circulating maternal PBMCs during pregnancy by determining the percentage of cells carrying proviral DNA by in situ DNA-PCR and viral burden by plasma quantitative branched chain DNA signal amplification. Further, we will determine the relative transcriptional activity of those infected cells by examination of HIV-1 specific RNA (spliced and unspliced) by reverse transcriptase in situ PCR, in situ hybridization, and branched chain DNA amplification. Similarly, (ii) we will analyze the viral burden within placental tissue from the corresponding mothers in conjunction with histologic typing within the various placental layers, especially at the chorioamnion. Finally, we will determine the origin (maternal versus fetal) of infected cells by simultaneous tissue typing of infected cells by in situ DNA PCR for HLA- DQA and HLA-DRB1 antigens. 2. Determination of viral burden in newborn PBMCs in relation to disease development. We will analyze cord blood, plasma as well as circulating PBMCs from the newborn to determine proviral burden and transcriptional activity of infected cells at the time of delivery as well as throughout the first nine months of life by the techniques outlined above. In addition, we will determine the origin of any infected cells by HLA typing as demonstrated above to explore the potential for transplacental egress of maternal cells into the fetal circulation as a mechanism of in utero transmission of infection. Finally, we will follow the level of proviral burden within the newborn to determine its correlation with subsequent disease progression. If such a correlation is established, these techniques may be used as a means of early diagnosis of infection in the newborn.